Poor Barack Obama. The heads of dysfunctional relatives are poking out of the branches of his family tree. It turns out Obama is Dick Cheney's eighth cousin. This is according to Cheney's wife Lynne. She claims that if you go back eight generations, they share an ancestor. This revelation follows a similar disclosure last month that Obama is also related to George W Bush: they are 11th cousins.

What does this prove? Lynne Cheney sees it as 'an amazing American story that one ancestor...could be responsible down the family line for lives that have taken such different and varied paths as Dick's and Barack Obama's'. But it doesn't demonstrate anything at all, other than the fact that if you go far enough back in history, you can relate anyone to anyone.

If you are seeking enlightenment, a family tree is no more useful than a horoscope. To see anything more in it is sentimental and narcissistic. Everyone's lives follow 'different and varied paths', regardless of their origins. It would be very strange if you shared the same political beliefs as your 17th-century ancestors, let alone their personality traits or foibles.

The all-knowing bloodline is a common fantasy, though. Amateur genealogists assume that family trees reveal the truth about the self. This is the thrill audiences are looking for in BBC1's Who Do You Think You Are?. The money shot is the expression on the celebrity's face when the researchers dig up an unexpected lineage.

The ultimate came in last week's episode, featuring rowing champion Matthew Pinsent. His tree traced back to Edward I. According to medieval genealogy, monarchs are divinely appointed, so this meant Pinsent was actually a direct descendant of Jesus. But that wasn't all. 'At the top of your pedigree,' he was told, 'there is God.'

Following one branch of your family tree is not the key to who you are. It's just a good anecdote. With genealogy, you can just pick out the bits and pieces that appeal to you and ignore the rest. As with a horoscope, some of it might have significance, but it could also be true for anyone. The Almighty could feature in any of our family trees. On the other hand, so could Bush or Cheney. Chainsaw, anyone?